The present invention relates generally to vibration sensors and more particularly to vibration sensors adapted to detect vibrations which may originate from a combustion in a chamber of a liquid cooled internal combustion engine. One type of such vibration is commonly called knock, ping, or detonation. Generally, the signals sought are those resulting from and which are diagnostic of combustion phenomena.
One useful purpose for such a device is taught in applicant's U.S. Pat. No. 4,116,173, issued Sept. 26, 1978, entitled "Internal Combustion Engine Ignition System" and the disclosure of this earlier Patent is hereby incorporated in the present application by reference. According to principles taught in this earlier Patent, the output of a detonation sensor may be filtered and used to automatically and incrementally retard and advance the spark advance of a spark ignited engine, operating the combustion chambers by closed loop control to avoid objectionable detonation, yet at spark advance angles to obtain maximum energy from fuels of various octane ratings with which, from time to time, the engine may be supplied. This invention is particularly well suited to automotive vehicle applications where the engine is almost continuously subject to changes in speeds and loads.
Prior art detonation sensors suitable for this purpose divide generally into two groups.
First, there are those which measure combustion chamber gas pressure. These are commonly called pressure transducers or engine indicators, and may be capable of sensing the extremely rapid rise in pressure which is indicative of detonation in the combustion chamber in which they are installed.
The second group comprise sensors which are rigidly mounted to the engine cylinder head, block or manifold and which respond in some way to accelerations or vibrations of their specific mounting location. These are commonly called accelerometers.
The principal disadvantage of the combustion chamber pressure transducer approach is that one sensor is required for each combustion chamber resulting in excessive cost and complexity in the installation and wiring.
The second or accelerometer approach does not require a transducer for each combustion chamber as, from a single mounting location, the result of high intensity detonation may be heard from occurances in any cylinder of the engine. A further advantage of such a transducer is that it may be time shared so as to correctly attribute the result of a detonation in any of the several combustion chambers to the correct portion of the signal output stream from such a transducer; this was first taught in applicant's above mentioned Patent. The rigid metallic connection between the combustion chambers and the sensor produces extremely strong output signals in response to detonations. It is well known that the mounting location for such a sensor must be carefully selected as to produce roughly equal outputs from detonations in any of the several combustion chambers of a multi cylinder engine.
Generally, in closed loop detonation control systems which retard spark advance as a result of a sensor output, the system must make continual decisions that detonation has or has not occured. The ability of the system to discriminate between signals resulting from detonation and signals resulting from other sources as from vibrations from normal operation of the engine's mechanical valve train, is of prime importance. The more such a system can advance the spark timing without encountering objectionable detonation, the greater are the increases in fuel economy and engine power which can be obtained.
Too advanced a spark produces detonation which may be objectionable to the ears of a vehicle driver, results in a substantial loss in power output, and, if prolonged, can damage the engine.
It is well known that, if the sensitivity of such a system is set such that, say, valve train vibrations are occasionaly mistakenly accepted as indications of detonation, a "false retard" condition ensues and the excessively retarded spark may result in a loss of vehicle performance and fuel economy.
It is apparent to the applicant that the detonation sensing problem is that of detecting a signal in noise. The vibrations produced by an actual detonation of a charge are the signal sought, and must be found with a high degree of certainty even when these signal vibrations are intermingled with other vibrations which may have similar characteristics and which may arise from other sources or events.
Every location on the rigid structure of an engine block, cylinder head and manifold where an accelerometer type detonation sensor may be mounted carries a portion of the internally generated stresses incident to rotation and translation of engine parts as they carry out their normal functions. Due to the complexity of the rigid structure it seems reasonable to believe that every location carries a unique share of these vibratory stresses and that these stresses will vary under various engine operating conditions. Lack of uniformity in manufacturing processes, such as casting wall thicknesses also may produce differences in response at the same location for different engines of the same design.
In contrast to the individuality of noise characteristics of various mounting locations for detonation sensors on rigid structural parts of the engine, applicant has discovered that the sensing of detonation from a vantage point elastomericaly isolated from operating stresses of the engine rigid structure, yet hydraluically coupled to said structure, provides significant advantages as to distinguishing the desired signal from undesired noise. The cooling system of a liquid cooled internal combustion engine offers a number of such vantage points. The distributed nature of the cooling system throughout the engine cylinder head and block assures that all vibrations generated will be communicated to the liquid coolant. The efficient sound transmission through the coolant assures that these vibrations may be made available at substantial distances from the rigid engine structure. The large diaphragm like area of the cylinder head portion of the combustion chamber enclosure actually can flex, intruding slightly but suddenly into the coolant passages which are cast into the cylinder head and assures that the practically instantaneous step function resulting from detonation will be massively communicated to the engine coolant.
This invention provides an engine control system which senses the vibrations resulting from detonation from vantage points or locations associated with the cooling system of a liquid cooled internal combustion engine.
This invention provides an engine control system which senses vibrations having a characteristic which is diagnostic of a combustion of a charge in the combustion chamber of an internal combustion engine from a location isolated from and hydraulically coupled to the main rigid portion of the engine structure.
This invention provides an engine control system wherein the tuning of a signal to a sensor diagnostic of a combustion in the combustion chamber of an internal combustion engine to a particular frequency is accomplished by varying the length of a hydraulic passage.
This invention provides means to combine a detonation sensor with a portion of a liquid cooling system of an internal combustion engine to achieve advantages in sensing detonation and to effect economies in construction of the combined result.
This invention provides an engine control system in which the signal to noise ratio of the output from a vibration sensor for sensing vibrations diagnostic of a combustion in a combustion chamber of an internal combustion engine may be increased by incorporating sound absorbing materials and/or chambers coupled to the liquid coolant of said engine.
Further objects of this invention will be obvious from the following description.